Heritability and Risk of Inheritance in Multifactorial Disorders. Heritability may be defined as the additive proportion of total phenotypic variance in a polygenic trait attributable to genetic variation in a population. The values of heritability estimates can range from 0% to 100%. Heritability is a function of genetic factors, environmental factors, and the interaction of genetic and environmental factors. Simple Mendelian traits have a heritability of 100%. Diseases with a heri-tability of less than 100% are inevitably controversial, such as those listed in the previous paragraph.
Other non-Mendelian mechanisms of clinical significance have been recently re-evaluated in terms of new molecular knowledge (the "new genetics"), but are not considered in this review (Anonymous, 1997a, 1997b). These mechanisms include: X inactivation, dosage compensation, epistasis, ectopic recombination, meiotic drive, position effects, microdeletions, other contiguous gene syndromes, chimerism, pleiotrophic homeotic mutations, slippage, and germline mosaicism.
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